Series II N/A. How expensive is it to replace and how hard is it to repair?

If you do it yourself, it is not terribly expensive, depending on how in depth you plan to get with the repair. If you plan to have a machine shop go through the heads for you, that will easily be the most expensive part. The rest is gaskets and time, and, if needed, tools. This is based on my experience with a 2.8L V6. I have never had to tear too deep into a 3800, but they don't look any more complicated.

To have a shop do it will be expensive. Most of that will be their labor costs, though.

Quick search of Advances website

Head Gasket 20
Head Bolts 32
Lower Intake gasket 63
You'd still need upper and TB ~45

Then it'* all labor. Depends on your comfort level. The upper intake and lower intakes would need to come off. Depending on which head (front/back) will determine the next. A rear (by firewall) head would require the unbolting of the accessory setup from the head only.

Not horrible job..will take some time though.

Check your diagnosis first. Head gaskets are extremely rare, and the symptoms may LOOK like the proper diagnosis, but it'* generally some other gasket or failure.

Well, I found this and was just pondering a bit...
http://seattle.craigslist.org/car/160828559.html

Have they ocnfirmed it is actually the head gasket. Could be just the UIM. Shoot them an email and ask how they came to that conclusion.

If it was the UIM or gaskets and was neglected, you'd better just plan on replacing the engine.

Agreed, its kinda rare for the 3800 to blow head gaskets with the cast iron head and block. Upper and lower intake gaskets are suspect and like mentioned earlier, if they were neglected and coolant got into the oil for a period of time the motor could be toast.

I bought a '97 SE in non-running condition. I replaced one head, the head gaskets, and the UIM and all associated gaskets. It'* not THAT hard, but does require a bit of mechanical knowledge and experience. By the way, mine is a loaded SLE, with 104k one-owner miles, and I paid $800.

It'* not that bad but if you've never done it before it could take a weekend.
Some things i've noticed..
Don't unhook the fuel rail from the line and TB from the UIM cause you don't need to.
Everything is easy down to the heads. Then you have to mess with rusted exhaust manifold bolts(great for shaving skin off your knuckles) and prepping the block and head. I used brake cleaner I think, a few fresh razor blades, and a red scotch brite pad.
Make sure to run a tap through the head bolt holes a few times to get all the crap out of there. Bill Wren gave me the tip of dabbing a little oil on the head bolts to help distribute the torque on the threads.
Leave the oil in there untill your done then drain it so everything that fell to the pan can get out easier. Then change it!
Its much easier to put the heads in with 2 people on opposite sides of the car.
You can re-use the UIM gasket and TB gasket too.